Yes, GM did install a trailer hitch wiring harness, but my experience was it was very much a 'Wheres Waldo" search.
If in interpret the diagram, the trailer wiring harness should be located just behind the rear axle, favouring the passenger side. I hope it is on your van, because neither I nor the GM dealer could find it. Their suggestion was to open up the interior and tap into the main harness.
Once I pulled off the drivers sidewall, I found a 5or6 pin connector located about halfway back, around the vicinity of where the middle seats are. It had the running lights plus ground and a 12V source. I tapped into those wires, running them into the rear power compartment, where I could coil the wires and connector. This allows it to be stored without being damaged and feed the connector out through the rear hatch when needed
Edit:
diagrams did not show up unable to post them